Multi-wavelength Light Beam Induced Charge (LBIC) mapping of diffusion length is a known technique for determining minority-carrier diffusion length. However, the technique's slow speed and restriction to finished solar cells (wafers cannot be measured) are some of its main limitations.
Images of minority carrier effective lifetime in silicon wafers can be obtained using the techniques of Carrier Density Imaging (CDI) or Infrared Lifetime mapping (ILM). The conversion of images of minority carrier effective lifetime obtained with these methods into images of diffusion length requires additional knowledge of the silicon wafer, for example, the spatially dependant carrier mobility and the spatially dependant surface recombination velocity. A significant limitation of these techniques is that such techniques cannot be applied to finished solar cells with a full rear metallization, because the techniques are based on infrared free carrier emission or infrared free carrier absorption, measured against a cold or heated background, respectively. Thus, these techniques are ruled out for industrial screen printed solar cells. In addition, CDI and ILM are affected by artifacts resulting from excess carriers in space charge regions and by artifacts resulting from minority carrier trapping.
Electroluminescence images of solar cells have been used by Takashi Fuyuki's group to get diffusion length images: Fuyuki, T., H. Kondo, T. Yamazaki, Y. Takahashi and Y. Uraoka, “Photographic surveying of minority carrier diffusion length in polycrystalline silicon solar cells by electroluminescence”, Applied Physics Letters, 2005, 86(26), p. 262108. This technique is limited to finished cells. The measured photon count per pixel is assumed to be linear in the diffusion length. A separate calibration is required to obtain a linearity factor that converts counts per pixel into diffusion lengths. Also, the influence of photon reabsorption and of resistive losses due to lateral current flow is neglected in this simplified analysis, effects that can lead to significant experimental artifacts.
Thus, a need clearly exists for improved methods and systems for determining the diffusion length of minority carriers in semiconductor structures.